In magnetic resonance imaging, part of the procedure is to apply radio frequency pulses to resonating nuclei to obtain selected tip angles. The tip angles most commonly used are a 90.degree. angle and a 180.degree. angle. Low tip angles such as between 5.degree.-90.degree. are also routinely used. If the radio frequency pulses produce a tip angle which differs enough from a selected tip angle, the difference or error will cause degradation in the result of the magnetic resonance procedure; i.e. reduced signal to noise ratio in the images.
In magnetic resonance imaging each patient loads the system differently and therefore the energy of an RF pulse which excites an RF coil to produce a selected tip angle of say 90.degree. for example is changed per patient. Accordingly, the RF pulse has to be calibrated for each patient to determine how much RF power must be provided to produce a 90.degree. tip angle. In the prior art, there are many patents disclosing methods and means for calibrating the tip angles. See for example, U.S. Pat. Nos. 4,983,921, 4,866,386 and 4,814,708.
In general, the art calibrates RF pulses by determining the magnitude of an RF pulse which provides maximum or minimum values of the signal amplitude. It used these measured magnitudes for calibrating the RF pulse required to provide a desired tip angle. Different RF pulses are used and the signal resulting from irradiation by the RF pulses measured. The magnitude of the RF pulse that results in the maximum signal is determined to be the RF pulse which gives a 90.degree. tip angle. However, as is well known, the signal amplitude depends on the RF amplitude in a generally sinusoidal manner with relatively broad maximum and minimum areas. Accordingly accurate determination of the maxima and minima are difficult to obtain. The magnitude of an applied RF pulse, that is the area under the RF pulse, determines the tip or nutation angle.
The use of the signal amplitude to determine the tip angle is problematic for other reasons as well. Among other things, the signal amplitude is strongly dependent on T1 and T2 decay time characteristics of the subject being examined and is sensitive to motion of the subject, be it flow, respiration or body movement.
In general, the nutation of the spins in the patient to be imaged is typically provided by applying the RF pulse to an RF coil that is closely coupled to the patient. Thus, RF coil loading and hence the resulting RF amplitude depends on the position, size and other parameters of the patient. Therefore, it is typically necessary to recalibrate the RF pulse at least for each patient. Even if the patient (or coil) is moved the Q factor and loading of the coil changes, so that the amplitude of the RF energy to the coil must also be changed. This typically requires the RF pulse to be recalibrated. Each calibration takes valuable throughput time during which the patient is within the magnet and subject to the discomforts thereof and during which the device cannot be used for additional patients.
Thus, in order to increase patient throughput, to reduce the amount of time that the patient is in the magnet and to minimize the apprehension that some patients suffer from being within the magnet, it is important to perform the RF calibration as rapidly as possible.
In addition to being inaccurate, the prior art systems are generally time consuming because many iterations are required to determine the RF pulse magnitude based on the maximum tipped signal.
In order for the magnetization to recover following an excitation and before the next suppression pulse, the time-to-repeat (TR) should exceed T1. The iterations with the long TRs cause the calibration time to be long. Accordingly, a calibration system is required which is basically independent of the relaxation times, is relatively insensitive to motion and wherein calibration of the RF pulse can be performed within time periods in the order of seconds.